The Ellipsoidal Reflector Spotlight (ERS) and the Parabolic Wash light (PAR) are two of the most popular lighting fixtures used in theatre, television, and architectural lighting. An ERS employs a reflector generated from an ellipsoidal or near-ellipsoidal curve rotated about the longitudinal axis of the optical system to define a reflecting surface, typically referred to as an ellipsoidal reflector. An ERS also produces a beam with a sharp edge, which, if projected on a flat surface, results in a ‘spot’ of light.
In a PAR optical system, a parabolic or near-parabolic curve is used to define a reflecting surface, typically referred to as a parabolic reflector. A beam exiting a parabolic reflector is substantially parallel to the optical axis of the PAR system. That is, the light beam is made up of light rays that are substantially parallel to each other and to the optical axis. Several such light beams may be used to ‘wash’ a target in light, where the beams overlap without the edges of individual beams being distinguishable.
FIG. 1 presents a schematic cross-section view of a prior art ERS optical system 100. A lamp 102 is mounted in an ellipsoidal reflector 104. The lamp 102 and the reflector 104 each have a longitudinal axis, which are coincident and define an optical axis 120 for the ERS optical system 100. The reflector 104 has a rim 105 forming an aperture from which emerges a light beam 106. When the lamp 102 is positioned adjacent to one of the two foci defining the ellipsoidal or near-ellipsoidal curve used to generate the reflector 104, the light beam 106 converges to a narrow diameter at the second focus of the reflector. In the ERS optical system 100, a projection gate 108 is located adjacent to this second focus. The projection gate 108 may simply be a circular aperture, or it may contain a light pattern generator 110.
Light rays of the light beam 106 cross over the optical axis 120 as they pass through the projection gate 108, resulting in diverging light beam 112. The light beam 112 is converged by a projection lens 114 to form light beam 116. The projection lens 114 projects an image 118 of the light pattern generator 110 located in the projection gate 108. If no light pattern generator is present, the projection lens instead projects an image of the projection gate 108 itself. The projected image of the projection gate 108 or the light pattern generator 110 comes into focus at a distance from the projection lens 114 determined by several optical properties of the optical system 100. By repositioning the projection lens 114 along the optical axis, the resulting image can be made to be in focus at various distances from the projection lens 114, resulting in a beam with a sharp, or hard, edge.
A PAR optical system, in contrast, may consist solely of a parabolic reflector and lamp, although a lens may be placed after the reflector to further smooth or shape the beam. A PAR optical system does not project an image and is therefore referred to as a non-imaging optical system. The edges of a light beam produced by a PAR optical system are not sharp and may fall off quite gradually, resulting in a soft-edged pool of light.
An ERS optical system may alternatively be designed to produce a soft-edged wash beam. If a non-imaging lens, such as a stippled Fresnel lens, is employed in place of the projection lens 114, the light beam produced is substantially parallel to the optical axis 120 of the optical system and the edges of the light beam are softer. Typically, the user of a wash light fixture desires that a large diameter light beam exit the lighting fixture, requiring that such a non-imaging lens be placed at a greater distance from the projection gate 108 than the projection lens 114, where the light beam 112 has diverged to a suitably large diameter. Thus, an ellipsoidal wash light fixture of this design is typically longer than an ERS spot light fixture employing the same ellipsoidal reflector. An ellipsoidal reflector whose second focus is closer to the rim of the reflector may be used to reduce the length of an ellipsoidal wash light fixture of this design.
In another alternative, in order to soften the edges of the beam of an ERS optical system, diffusion, or scattering, of the light beam may be introduced at some location in the optical system. This diffusion may be placed in the beam manually, as part of preparing the light for use. Alternatively, the diffusion may be inserted and removed from the beam by a motorized mechanism, controlled by an operator from outside the light fixture. However, such diffused beams are often not considered by users as a suitable replacement for a beam from a parabolic optical system or an ellipsoidal optical system with a non-imaging lens.
Wash light fixtures may also be designed around reflectors of types other than ellipsoidal and parabolic reflectors. For example, a symmetric reflector may be generated by rotating about the longitudinal axis of the optical system a segment of a curve defined by a mathematical function other than an ellipse or parabola, or a segment of an arbitrary curve. Other reflectors may have a non-circular cross-section designed to smooth the irradiance distribution of light beams generated from lamps having an asymmetric intensity distribution.
In the design of any wash light fixture, at least two challenges are encountered. First, a small overall size for the fixture is desired in order to allow more fixtures to be placed in an available space, and, in the case of remotely controlled motorized fixtures, to reduce the size and power requirements of the motors and mechanisms. Second, while a large beam size from the fixture is generally desirable, the materials used to filter the color of the light beam in the fixture may be expensive, leading to a desire to minimize the amount of filter material used in each fixture.
A theatrical, television, or architectural lighting system typically includes both spot and wash lights. As a result, a company manufacturing or renting lighting systems typically maintains an inventory of both types of light fixtures.
FIG. 2 depicts a schematic cross-section view of a prior art ellipsoidal reflector spotlight 200. A lamp 202 and ellipsoidal reflector 204 project a light beam through a projection gate 208. A projection lens 214 forms an image of the projection gate 208 at a distance from a front aperture 236 of the ERS 200.
The lamp 202 and ellipsoidal reflector 204 are enclosed in a reflector housing 230 to form a light beam generator. Attached to the reflector housing 230 is a lens barrel 232, which encloses the projection lens 214 and the projection gate 208. A coupling mechanism 234 may allow the lens barrel 232 to be removed from the reflector housing 230 and to rotate about an optical axis 220 of the ERS 200. This rotation permits a light pattern generator installed in the projection gate 208 to be aligned at a desired angle.